1. Field of the Invention This invention relates to vibration damping of bearing assemblies in turbomachinary, and more particularly to bearing squeeze film dampers.
2. Description of Related Art
Turbomachinary such as high speed gas turbine engines employ bearings to support the rotating shafts of the engine. Typically a ball bearing is used to carry thrust, and the remainder of the bearings are roller bearings. Oil is a fluid typically used to lubricate and cool the bearing and bearing assemblies. A major problem facing designers and manufacturers of high speed turbine engines is the vibration and dynamic loads caused by rotor and shaft unbalance and by self-excited whirl (i.e. dynamic instability). Higher shaft speeds and the use of long slender shafts to drive large forward mounted fans in modern multi-spool gas turbine fan jet engines have made the problem more acute.
It is well known in the turbomachinary field to use squeeze-film bearing dampers to reduce the adverse vibrations due to high-speed rotor dynamics. Using a typical rolling-element bearing as an example, the outer race of the rolling-element bearing is fitted loosely in a bearing support housing, with a radial clearance which is a small percentage of the bearing radius depending on the design. Radial and orbital motion of the rolling-element bearing outer race is permitted but rotation is prevented by some type of mechanical restraint. The annular clearance thus formed is kept filled with oil, a typical damper fluid, and hydrodynamic forces are generated by the motion of the bearing under the influence of unbalance or other excitation. The squeeze film acts as a nonlinear spring and damper system which if properly designed can significantly reduce the dynamic loads and suppress dynamic instability.
The occurrence of cyclical orbital motion of the shaft of a gas turbine engine rotor as a result of dynamic instability and rotor unbalance is well known, and fluid damping means such as squeeze film dampers are utilized to dampen orbital and other undesirable shaft motion. In general, such squeeze-film dampers use a pressurized fluid, such as an oil, in a small damper annulus between a bearing support element, such as the outer circumferential surface of a rolling element bearing outer race, and an opposing annular wall of an annular bearing housing chamber in which the race is confined with permitted limited radial motion. Damper fluid is introduced into the defined annular damper annulus between the circumferential outer race and the annular housing, and vibratory or orbital motion of the bearing race causes the race to exert a squeezing action on the damper fluid to move or distribute the fluid throughout the defined damper annulus.
Two types of squeeze film dampers, sealed and unsealed are generally known. One type of unsealed squeeze-film damper is disclosed in of U.S. Pat. No. 5,071,262 entitled "Squeeze Film Damper Fluid Control" and includes a piston ring for damper fluid sealing outboard of a fluid plenum groove adjacent to each end of the squeeze film annulus. The fluid plenum groove serves as a lower pressure receiver for damper fluid and as a reservoir to receive and supply fluid during pumping action of the squeeze-film damper.
The present invention addresses the sealed type damper. Typically, spaced apart opposite piston type sealing rings are disposed in annular grooves between the bearing race and the housing as axial fluid seals for the damper annulus. The orbital motion of the bearing race causes the race to exert a rotational squeezing action on the damper fluid which causes a pumping action that generates a lower pressure in the squeeze film annulus on the suction side of the pumping cycle. This action causes the squeeze-film damper to suck air in from the sump through paths across the end seals on the low pressure side of the squeeze-film damper. This can cause air entrainment in the oil which has been found to significantly reduce the squeeze-film damper performance and result in erratic damper behavior. This ingestion of air in the damper fluid is a condition referred to as gaseous cavitation (air entrainment) which is to be distinguished from vapor cavitation (oil boiling in the low pressure side of the squeeze-film damper).